Final answer:
The option that is not a fibrous protein is the actin filament. Actin forms microfilaments, whereas collagen, elastin, and α-keratin are fibrous proteins involved in structural support.
Step-by-step explanation:
The student has asked which of the following is not formed by fibrous proteins: collagen, actin filament, elastin, α-keratin, or extracellular matrix. Among the options listed, the one that is not a fibrous protein is actin filament. Actin is a globular protein that forms microfilaments, which are part of the cytoskeleton of cells and are involved in processes such as muscle contraction and cell movement. In contrast, collagen, elastin, and α-keratin are all fibrous proteins. They have structural roles; for example, collagen and elastin are part of the extracellular matrix, providing tensile strength and elasticity, respectively, whereas α-keratin is found in hair and nails, contributing to their rigidity and durability. The extracellular matrix itself is a complex mixture of proteins, carbohydrates, and other molecules, but it is not a protein.